The invention relates to charged particle emitting assemblies and emitters. The invention is concerned for example with the generation of high power electron beams (EB) and transmission into vacuum chambers operated at pressures in the range approximately 10xe2x88x921 mbar up to several hundred millibar. However, the invention is also applicable to other types of charged particle beams including those defined by negatively and positively charged ions. For convenience, only electron beams will be discussed.
Electron beams are readily produced by release of free electrons from an emitter and acceleration in an electric field. For electron beams which are merely used for applications such as vacuum melting of metals, beam quality in terms of energy density distribution, beam brightness and beam profile, is of little importance. Typically, xe2x80x9cbrightnessxe2x80x9d is defined as current density/steradian.
For other applications, beam quality is extremely important and moreover must be stable and reproducible. In the case of electron beam welding (EBW), for example, the ability to produce repeatedly deep narrow fusion zones of consistent depth and width is critically dependent on:
i) the beam energy density distribution
ii) the position of focus with respect to the workpiece surface, and
iii) the beam brightness which involves both spot size and convergence angle factors.
Ideally, for electron beam welding, it is important to achieve a clearly defined energy density distribution and usually this is Gaussian in form. Also, to perform deep narrow welding, the angle of convergence of the beam needs to be controlled within a relatively tight range. Certainly for welding of steels, for example in section thickness of 100 mm-150 mm, a beam semi-angle of greater than 1 degree leads to weld pool instabilities and internal defects. A beam which is near parallel, on the other hand, may be highly suited to welding such thick sections but is unsuitable for producing very narrow welds in steel sections of 1 mm-10 mm. In addition, in the case of the thinner section range, welding beam energy distribution is much more important. If for any reason the energy distribution includes a significant fringe, this is reflected in the weld fusion zone shape. Thus, instead of achieving a near parallel fusion zone as in the case of the Gaussian distribution, a much wider non-parallel fusion zone with a so-called xe2x80x9cnail headxe2x80x9d feature is produced. More beam power is required for the same weld depth, lateral shrinkage after welding is overall greater and because of the wider weld width at the top compared with the bottom, uneven shrinkage occurs resulting in distortion of the component as indicated. For precision components this is often unacceptable and may also lead to weld cracking.
Similarly, it is important, particularly for thin section welding, to achieve sufficient intensity in the focal spot. For systems which produce a near parallel beam, even without fringes, insufficient intensity leads to relatively wide tapered fusion zones accompanied by excessive distortion and again a risk of cracking. Near parallel beams are not necessarily focusable, space charge spreading can still occur in a vacuum environment even in spite of strong positive ion neutralisation effects. Thus, for a near parallel beam entering a focusing lens attempts to focus a beam over long distance result in little if any reduction in the beam diameter. Indeed, the beam profile and intensity characteristics can often at medium and high powers, be totally dominated by ion-electron interactions.
It is, therefore, very important to launch the beam from the electron gun with a well defined divergence (within a specified range), high brightness, low aberration and without fringes.
One possible means of achieving higher convergence angle to combat space charge spreading in the medium to high beam current range with a triode gun is to employ electrodes which produce a more strongly focusing field. This, however, leads to excessive convergence at low current when the grid field becomes an additional powerful focusing element. Large swings in convergence angle are generally undesirable even for high vacuum EBW and present greater difficulties in a system which employ a beam transfer system for reduced pressure (5xc3x9710xe2x88x921 to xcx9c250 mbar) or non-vacuum (xcx9c1000 mbar) operation where fine bore nozzles are employed to restrict gas leakage into the gun region.
Yet another method of achieving greater convergence, to combat space charge spreading at high current levels, is to design a gun in which the cathode, grid electrode and anode are placed in close proximity. This leads to more rapid acceleration of the electrons over a shorter axial distance reducing the possibility of mutual electron repulsion. Unfortunately, such an arrangement increases the electrical surface stresses on the electrodes and can lead to increased high voltage breakdown tendency.
Avoidance of beam fringes and optimum focusing of electron beams is extremely important when the beam must be transmitted through narrow orifices in order to extract the electrons from the high vacuum (5.10xe2x88x925xe2x88x925.10xe2x88x926 mbar) region of a gun housing into working chambers operating over the approximate pressure range of 5.10xe2x88x922 to 1000 mbar. Here, the rate of leakage of gas from the working chamber into the gun housing is primarily determined by the diameter and length of the orifices apart from the number of orifices and the pumping capacity of the interstage pumps.
Beam fringes tend to contain large amounts of power and even for low total power operation (e.g. 5 kW), the ability to absorb this extraneous power on the orifice nozzles is limited even if substantial water cooling is applied; unlike electron microscope devices, where the beam power is extremely small, it is impractical to strip off the unwanted fringe on interception diaphragms. For similar reasons, it is important to avoid a low brightness, near parallel beam because of the large beam diameter.
Beam quality and whether or not a particular electron gun produces a pure un-aberrated beam with a well defined divergence is very dependent on gun design and particularly the cathode design and the detailed geometry of the electrodes in the immediate vicinity of the cathode.
Most electron guns used for EBW are triodes. The use of the grid electrode ensures that at low beam current, cathode emission is limited to a central portion of the cathode but the presence of the strong electric field created by the grid leads to considerable beam aberration.
The outer electron trajectories have a shorter focal length in the strong grid field because they are closer to the edge of the grid cup hole than the more central electrons. Also, as the grid voltage is reduced to increase beam current, the emission area expands and may even permit electrons to be released from the cathode edges where adverse geometrical features produce electron trajectory flight paths radically different to the main electron flow. In addition, the weakening grid field combined with increased space charge in the beam, as the beam current is increased, can result in gross spreading of the beam and loss of primary focus. Also, the primary focus waist and the virtual image position (apparent to the first focusing lens) can move considerable distances up and down the beam axis dependent on beam current level.
Beam fringes produced by such a gun, the drift of primary focus with beam current, the lack of beam convergence angle at high current and the relatively high convergence angle at low current, can adversely effect welding performance for even conventional systems projecting beams into relatively high vacuum chambers (5xc3x9710xe2x88x923 mbar). For beams which need to be transmitted through small orifices, operations can be difficult or even impossible, particularly for high power (greater than say 30 kW) operation.
In accordance with a first aspect of the present invention, a charged particle emitting assembly comprises an emitter member for emitting charged particles of one polarity; a tubular shield electrode circumferentially surrounding the emitter member and held in use at the same polarity as the charged particles; and a tubular accelerating electrode positioned substantially coaxially with the shield electrode and held in use at the opposite polarity to the shield electrode, the arrangement being such that charged particles from the emitter member initially spread laterally outwardly and then are focused into a beam which passes through the tubular accelerating electrode.
The invention involves a special diode gun construction. A diode gun, compared with a triode, has many distinct advantages including:
i) it is possible to virtually eliminate aberration,
ii) control of beam shape and quality at high current is more readily achieved than with a triode,
iii) sufficient beam convergence can be achieved at high current without excessive convergence at low current,
iv) under gun discharge conditions, when operated in the temperature limited mode, the beam current does not surge, unlike a triode gun where breakdown of the high voltage between the gun electrodes and ground precipitates shorting of the grid supply and instantaneous release of full beam power,
v) the diode gun requires fewer auxiliary supplies (two in the case of a conventional indirectly heated diode; one if an RF excited indirectly heated diode; one if a directly heated diode), and
vi) for a diode gun, the electrical cable and connections are simpler, particularly for an RF excited diode where only one HT connection is required and no auxiliary supply connections; the RF power is inductively coupled from a remotely spaced high frequency aerial or primary winding positioning within the gun housing.
For a diode gun, in the absence of the focusing action of the grid field, especially with a small diameter cathode, the high space charge density in the beam, particularly at high current levels, can result in spreading of the beam and lack of a well defined primary focus; indeed the beam may be insufficiently collimated to even pass through the anode hole. Of course, one solution to avoid excessive beam spreading would be to employ a large diameter cathode but this would fundamentally reduce beam brightness and complicate the equipment and increase the costs.
In the invention a distinct swell on the electron beam is created initially thus creating an artificially large beam source which can then be subsequently focused with a relatively high angle of convergence by the main cathode/anode electric field in the inter-electrode gun.
As regards the ultimate beam brightness which can be achieved in the beam for a given accelerating voltage, this depends on many factors, but for high power EBW guns it is very dependent on cathode diameter and gun design. Fundamentally, it is very important to minimise cathode diameter, since for a given beam convergence angle and operating voltage, anywhere in the post gun focusing system, the spot size is proportional to cathode diameter and proportional to the square root of the cathode operating temperature in degrees Kelvin.
The invention enables cathode diameter or more strictly emission diameter to be limited and so improves beam brightness. Moreover, limiting cathode diameter and cathode total surface area as well as operating temperature, reduces the auxiliary heating power requirements, auxiliary power supply costs and also minimises gun operating temperature and electrode geometry thermal distortion effects. Reducing gun heat input in turn reduces cooling requirements which invariably present difficulties for an EBW gun suspended in vacuum on the extremity of a long, high voltage insulator since the insulator is not only a poor conductor of electricity but also of heat.
Naturally, the cathode emission area must be adequate to produce the required beam current for a given cathode life, since ion erosion, oxidation and evaporation rates increase with cathode temperature; but much can be gained by optimising the conflicting emission density and beam brightness factors.
In one example of the present invention, beam focusing in the gun region in the presence of heavy space charge loading is achieved by means of a deeply recessed cathode shield electrode combined with a long small diameter anode, the extremity of which is positioned close to the end of the cathode shield electrode or even well inside it. This creates a strong focusing action which works well at low, medium and high power levels.
The beam swell or lateral spreading can be achieved in a number of ways. One method is to select cathode diameter and electrode geometry such that the electron acceleration is initially relatively slow, enabling space charge spreading to produce strong radial outward motion. This is achieved by setting a relatively small cathode inside a deeply recessed cathode shield cup. If the cathode diameter is too small, this limits the maximum beam current for a given accelerating voltage before the gun becomes space charge limited and the initial spreading actions can be so large that the subsequent convergent electrostatic field in the inter-electrode gap produced by the general cathode shield/anode geometride electrode form is insufficient to refocus the beam into a distinct waist or cross-over. On the other hand, if the cathode is too large, the emission density at the cathode is too low to produce the required initial beam expansion and consequently the beam has subsequently insufficient convergence to avoid spreading during the final period of acceleration. Thus, the combination of electrode shaping and cathode size is critical for a given accelerating voltage and power operation range to achieve the best performance.
A second method of achieving the artificially large electron source without the need for high current density at the cathode involves producing a suitable electrostatic field immediately in front of the cathode to cause the beam to diverge. This can be accomplished by mounting the cathode on top of a conical or cylindrical projecting member which stands proud of the base of the cathode shield cup.
The cathode can be in excess of 5 mm2 in area, permitting operation at power levels in excess of 100 kW.
As described above, the initial beam swell created by space charge spreading is enhanced by a suitable electrostatic field. This increases the size of the apparent source after initial beam spreading, allowing a highly convergent beam to be produced with a well defined focus over a wide current range. Yet another possible means of promoting initial beam expansion is to shape the cathode surface as described below so that at least the emitter is essentially convex or conical.
The first aspect of the invention can be used in welding apparatus of various types including vacuum chambers operating in the pressure range 5xc3x9710xe2x88x925 mbar to 5xc3x9710xe2x88x922 mbar. However, the invention is particularly suitable for use with welding apparatus operating at intermediate pressure ranges, 10xe2x88x921 mbar to several hundred mbar, and even at high pressures and non-vacuum.
Typical industry sectors which have identified the potential benefits of such equipment are thick section steel pipe producers, offshore and onshore pipe welders, nuclear waste disposal companies, power generation equipment producers and aerospace component manufacturers.
For many of these applications, the material wall thickness to be welded in a single pass is in excess of 15 mm and may be as high as 150 mm or more. In either case, the need to weld quickly demands beam power levels of at least 30 kW and in some cases up to 100 kW or more.
For all diode guns, including the ones described above, one major problem which has remained largely unsolved prior to this invention was the fact that the sides or edges of the cathode emitted unwanted electrons with uncontrolled flight paths. Numerous methods to prevent this occurring have been attempted spanning several decades of effort and many research and development teams. One of the simplest arrangements was described in the U.S. Pat. No. 3,878,424 (filed on Jul. 17, 1973) in which a planar diode was proposed to overcome the spherical aberration effect of the grid electrode. In this, oxide (e.g. barium-strontium-calcium) was packed into a hole in a refractory metal cathode xe2x80x9cheaterxe2x80x9d plate which could be heated by various methods. In another variant, oxide was coated onto the surface of refractory metal plate. By heating the plate to a temperature well below the emission point of the refractory metal cathode plate, strong emission of the lower work function oxide occurred, thus avoiding edge effects. The proposed device may well be suitable for producing low power beams for electron microscopes where the oxide coating or plug in a hole is only some 100 microns in diameter, but for high power EBW guns which are continuously subjected to ion bombardment, gases and particulate matter from the weld pool, oxide cathodes would be rapidly poisoned destroying their emissive properties. Also, in the case of the oxide film this is typically only 50 microns thick and would be rapidly eroded in an EBW system. Moreover, the planar cathode plate would distort, causing adverse and unpredictable changes in beam divergence and beam projection direction. Differential expansion between the two different materials could also cause cracking and spalling of the emitter.
In yet another attempt to avoid extraneous edge emission, Bull et al, xe2x80x9cAn electrostatic electron gunxe2x80x9d, Metal Construction and B.W.J. November 1970 2 (11), p. 490, produced a spherical electrode, indirectly heated diode gun in which perforations were placed around a refractory metal cathode electrode to confine emission to a central area. This gun, however, also suffered from thermal distortion of the central cathode region and the perforation allowed electrons to pass through from the primary back bombardment into the main beam causing further main beam distortion effects. Some reduction in the flow of primary electrons was achieved by insertion of additional electron barriers on the rear of the cathode shield but cathode distortion remained a problem.
We have considered numerous other methods of controlling edge emission.
Leakage of primary electrons into the beam can be prevented by mounting the cathode on a continuous conical member as will be described in more detail below.
Cathodes of a similar form have also been assembled from two materials exhibiting different work functions, where the emitter is made from a low function material such as lanthanum hexaboride, which is not easily poisoned during EBW, and on outer supporting structure made from a refractory metal such as tantalum. Such an arrangement is also described in EP-A-0627121. Similar arrangements have been separately developed and described in GB-A-1549127 but this particular gun was distinctly different to the present invention in many respects.
In these earlier developments in order to support the lanthanum hexaboride button it was necessary to place it behind a lip in the refractory metal holder. The lip still strongly disturbed the electric field and resulted in appreciable spherical aberration in which the outer electrons were, as in the case of triode guns, focused at a shorter focal length than those near the axis. Lip thickness can be reduced by careful machining or by placing a thin refractory metal washer in front of the cathode, but in both cases thermal distortion caused the lip to distort outwards leading again to extraneous electron emission from behind the lip or washer.
Another technique which has been partially successful is the coating of the outer annulus of the exposed low work function material with a high work function material. For example, the cathode made from say lanthanum hexaboride, was coated around its periphery and in an annular form on the front face with tungsten. Although initially promising, this technique suffered from loss of the coating by ion damage, oxidation and evaporation in service. It was also difficult to avoid damaging the thin coating during assembly. In addition, the lip of the holder, although imparting a reduced focusing effect, nevertheless still produced unacceptable aberration.
Yet another technique which can be applied is to braze the lower work function material button into the holder using a compound such as molybdenum disilicide. Achieving a high quality non-porous braze without contaminating the LaB6 material was problematic and in the best cases the braze material tended to crack in service due to repeated thermal cycling.
In accordance with a second aspect of the present invention, a charged particle emitter comprises an emitter member mounted in an aperture of a support member to which it is electrically connected, the emitter member having a lower work function than the support member whereby at a working temperature, the emitter member emits charged particles from an exposed surface characterised in that the exposed surface of the emitter member is set back from or preferably substantially flush with an outwardly facing surface of the support member surrounding the aperture.
In the preferred form of the invention, the low work function cathode material is machined to a xe2x80x9chatxe2x80x9d shaped form which snugly fits into a central hole. Conveniently, the emitter member is a close fit in the aperture of the support member.
Part of the support member and part of the emitter member may be correspondingly tapered.
Alternatively, or additionally, the emitter member may be secured to the support member by a clip which engages each member.
In particularly preferred arrangements, the exposed surface of the emitter member and the outwardly facing surface of the support member define a common plane.
In accordance with a third aspect of the present invention, a charged particle beam column assembly for mounting to an evacuated charged particle beam source chamber has a sequence of controlled pressure chambers, each having inlet and outlet apertures through which a charged particle beam can pass and an evacuation port for connection to a pump to enable the pressure in the chamber to be controlled, whereby the pressure in successive chambers increases in use and is characterised in that an evacuation port is connected to a downstream chamber via a conduit which extends within the assembly past at least one upstream chamber.
This enables rapid removal and insertion of a replacement gun column for maintenance purposes. Conventionally, multi-stage pumped systems contain side intrusions to apply the pumping between the various nozzle restrictions. This makes for a complex column geometry which cannot easily be inserted and withdrawn from the vacuum chamber. The innovative step of concentric pumping overcomes these difficulties and is particularly important for applications such as offshore J-pipe laying where time is of the essence because of the very high cost of the hire of the large laying barge and the fact that only one pipeline can be welded and laid at any one time.
In more detail, this facet of the invention relates to providing a series of vacuum or partial vacuum chambers or ports in a compact arrangement or apparatus. In particular, this invention can be applied to so-called non-vacuum or Reduced Pressure EB systems in which the gun head comprises a plurality of chambers, ranging from vacuum in the region of the cathode to near-atmospheric pressure at the output end where the beam emanates into the open environment. These chambers have to be maintained at appropriate partial vacuum pressures, which permit the electron beam to pass through. This invention could also be implemented with the last chamber at or even above atmospheric pressure, for example if underwater welding is to be performed.
Such apparatus is commonly awkward in construction in so far as it requires several different vacuum pumping lines connected to the respective chambers in the gun head, these connections limit the access in the region of the output beam. Not only is such apparatus bulky, but the restricted access in the region of the various chambers limits the efficiency of the vacuum pumping. Therefore, the several vacuum lines have to be of a relatively large bore so as not to further restrict and impede the vacuum pumping.
This aspect of the invention provides a relatively compact and slim design of such a sequence of chambers, for example, as used in non-vacuum and Reduced Pressure EB welding systems. The invention can also provide a plurality of chambers, especially in the output region of the gun head, which are less than typically 170 mm in diameter, at least for the lower (or output) half of the head assembly. The use of vacuum lines or pipes near the operating output of the gun head can be avoided and in addition means for efficient pumping of the chambers (which are preferably maintained at appropriate pressures ranging from near-vacuum to near-atmospheric) can be provided.
In one arrangement, the chambers are defined by a set of tubular sections located within an outer tube, each tubular section having a radially outwardly facing opening, the radially outwardly facing opening of each chamber being circumferentially offset from the radially outwardly facing openings of all the other chambers; and a set of axially extending dividing walls positioned between the tubular sections and the outer tube to form the respective conduits, each conduit connecting the radially outwardly facing opening of a chamber with a corresponding evacuation port.
Alternatively, the plurality of chambers may be arranged as a sequence of discs carrying appropriate orifices, the set being fitted within a common sleeve, which is segmented. Each segment permits access to its respective chamber or port with good cross-section, as is the case with the concentric arrangement.
Yet again, a combination of concentric tubular sections and segmented cylinders may be utilised to give appropriate efficient vacuum pumping at the pressures concerned, with suitable cross-sections of access to the port or chamber respectively.
The array of chambers can be readily dismantled for replacement of the orifices, which may become partially blocked due to metal spatter or may be damaged by interception with the electron beam. Thus, provision is made for assembly and disassembly of the concentric tubular sections (or cups) or segmented cylinders respectively, while still maintaining adequate alignment of the orifices to the axis of the beam. This arrangement also reduces the leakage path between the chambers operating at partial pressure and the surrounding atmosphere. It will be appreciated that in the concentric arrangement, the regions of higher vacuum are contained within regions of partial pressure and hence are not directly exposed to the surrounding environmental atmosphere this greatly diminishes the effects of any small leak in seals.
In the case of the concentric cup arrangement, each part may have an integral screw fitting onto a common base carrying corresponding threaded portions together with, if desired, xe2x80x9cOxe2x80x9d ring seals. The cups may be provided with vanes, spacers or the equivalent to maintain their relative concentricity in assembly.
Likewise for the segmented arrangement, the corresponding parts may fit together with compressible seals as well as being held mechanically at the appropriate spacing and concentricity. In all these arrangements, the vacuum pumping lines are taken essentially to the rear of the gun head away from the output beam through appropriate connectors to the respective segments or annular orifices of the compact assembly of chambers operating at differential pressures.